Polyesters, such as poly(alkylene arylates), for example, those prepared from diprimary alcohols, namely, from glycols and terephthalic or isophthalic acid, have been available for a number of years. They are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319 and Pengilly, U.S. Pat. No. 3,047,539, incorporated herein by reference. These patents disclose that the polyesters are particularly advantageous as film and fiber formers.
With the development of molecular weight control, the use of nucleating agents and two-step molding cycles, poly(ethylene terephthalate) has become an important constituent of injection moldable compositions. Poly (butylene terephthalate), as for example, poly (1,4-butylene terephthalate) because of its very rapid crystallization from the melt is uniquely useful as a component in such compositions. Work-pieces molded from such polyester resins, alone or combined with reinforcements, in comparison with other thermoplastics offer a high degree of surface hardness and abrasion resistance, high gloss and lower surface friction.
Stable polyblends of PET and PBT can be molded into useful unreinforced and reinforced articles. See Fox and Wambach U.S. Pat. No. 3,953,394 incorporated herein by reference.
However, the crystallization rates of the polyesters leave room for improvement, since it is well known in this field that faster crystallization rates improve injection molding performance, as evidenced by reduced post-mold shrinkage, faster mold cycle times, and lower mold temperatures.
It has now been discovered that not only the crystallization rate but that the percent crystallinity of poly(ethylene arylates) can be increased by incorporating therein a small amount of a polyethylene ionomer or an alkali metal salt of benzoic acid, as for example sodium benzoate, potassium benzoate, or lithium benzoate. This increase in the rate as well as percent of crystallinity is particularly significant in the case of PET and other polyesters containing PET. The process is applicable not only to unfilled polyester compounds but also to mineral or glass-filled, or fire-retardant containing polyester compounds and compositions; and to impact modified polyesters, as well as to blends of polyesters, such as poly(ethylene terephthalate)/poly(butylene terephthalate) (PET/PBT) blends unfilled or filled with the aforesaid components.